Effects of dislocation strain on the epitaxy of lattice-mismatched AlGaInP layers

• Composition fluctuation and surface-roughness due to the effect of misfit dislocation strain on indium incorporation is studied.
• A GaAs spacer layer between misfit dislocations and the LED active layer is shown to be effective at reducing composition fluctuation.
• Control over composition and surface-roughness will have an impact on the utility of lattice-mismatched LEDs, laser diodes and multi-junction solar cells.

Strain fields arising from a non-uniform distribution of misfit dislocations in an underlying compositionally graded buffer are shown to be sufficiently strong to modify indium incorporation in III-phosphide light emitting layers. Composition fluctuations (xIn±0.02) in lattice-mismatched (AlyGa1−y)xIn1−xP thin films with length-scales of 5–10 μm and a broadened light emission spectra are observed. Cathodoluminescence, photoluminescence and wavelength dispersive x-ray spectroscopies are used in this analysis to generate spatial maps of luminescence spectra and element distributions in metal-organic chemical vapor deposition (MOCVD) grown films. It is seen that these fluctuations due to misfit dislocations are hard to eliminate via growth-kinetics alone but can be lowered through the use of miscut substrates or spacer layers between the graded buffer layer and the active layer. A link between crosshatch surface-roughness and group-III atom distribution under group-V rich growth conditions in both AlInP and GaInP films is also demonstrated. In summary, the interaction of the dislocation strain field with the growth surface can affect the optical characteristics of lattice-mismatched LEDs even if the final threading dislocation density is low.